To be able to carry out shift operations in vehicles or vehicle drivetrains without interrupting the traction force, in change-under-load transmissions two so-termed change-under-load shift elements are actuated at the same time during shift operations under load. For this purpose, for example electro-mechanical actuating devices are associated with the two change-under-load shift elements, which comprise for each change-under-load shift element a separate actuator or a separate force control element.
If a change-under-load transmission is configured as a dual-clutch transmission, then during driving operation it is always the case that only one of the change-under-load shift elements is actuated or shifted into the force flow of the vehicle's drivetrain, while the other is disengaged from the force flow and is in a load-free operating condition.
Only during a change-under-load shift operation or during the load transfer from the shift element to be disengaged in the direction of the shift element to be engaged, is it necessary, during a certain part of the shift sequence, to actuate both change-under-load shift elements at the same time, since the transmission capacity of the change-under-load shift element to be engaged has to be increased during a traction upshift or a thrust downshift, whereas the transmission capacity of the shift element being disengaged is reduced to the same extent. During traction downshifts or thrust upshifts the procedure just described does not take place until the end of the shift operation.
To avoid compromising comfort during a shift and to prevent unnecessary friction losses in a change-under-load transmission during the later stages of a change-under-load shift operation, at the load transfer time point when the transmission capacity of the shift element to be engaged has a value such that a torque of the drive engine of the vehicle's drivetrain can be transmitted in substantial measure by the shift element being engaged, the transmission capacity of the shift element being disengaged, is reduced to a value at which, so far as possible, only a very small torque can still be transmitted by the shift element being disengaged.
In addition, the electro-mechanical actuation devices provided for actuating the change-under-load shift elements are designed in such manner that in the event of a fault, the two change-under-load shift elements are not simultaneously actuated to such an extent that powerful braking effects occur in the area of driving wheels of the vehicle's drivetrain due to a stressed condition in the area of the change-under-load transmission, which in some circumstances could give rise to situations critical in relation to driving safety.
The design of the electro-mechanical actuation devices with separate actuators for the change-under-load shift elements demands considerable control and regulation effort during changes under load, and entails high production costs.
Thus, the purpose of the present invention is to provide a transmission device with at least two shift elements that can be changed under load and methods for operating such a transmission device, by means of which shifts, in particular changes under load, can be carried out simply and inexpensively.